Many ore bodies comprise a mixture of valuable sulfide minerals with a number of non-sulfide minerals, including carbonaceous minerals (e.g. graphite, carbon-based residues as exist in Mt Isa, Australia ore bodies), talcose minerals (e.g. talc, brucite etc. which are associated with Western Australian nickel deposits and the Woodlawn, New South Wales, Australia base metal deposit) as well as amphiboles.
The non-sulfide minerals have naturally hydrophobic characteristics. The degree of hydrophobocity varies according to mineral and ore type from weakly hydrophobic to strongly hydrophobic. As a result, these so-termed "gangue" minerals have a tendency to float and are very difficult to separate from other valuable minerals, notably the sulfide minerals, e.g. chalcopyrite (CuFeS.sub.2), pentlandite ((Ni,Fe).sub.9 S.sub.8) and sphalerite (ZnS)). When present in mineral concentrates, these "gangue" minerals often attract penalty charges at the smelter and, indeed, may be the cause of rejection of the ore concentrate by the smelter.
In practice, two approaches to this problem exist, namely to minimize the flotation of the non-sulfide "gangue" minerals using specific reagents or, alternatively, to encourage flotation of the "gangue" minerals in a pre-flotation step prior to the flotation of the desired minerals.
In the first approach, reagents such as depressants (guar gum, carboxy methyl cellulose and the like) or dispersants, e.g. sodium silicate, are employed to minimize the flotation rate of the non-sulfidic minerals. In some cases, for example with copper-nickel-iron bearing ores, nitrogen is used as a flotation gas in combination with organic depressants. This tends to strengthen pyrrhotite depression and increase nickel recovery. While successful to some extent, the use of these organic depressants is non-specific and adversely affects the flotation behavior of the sulfide minerals in terms of metallurgy as well as froth structure. In addition, the use of such reagents is costly and, if it were possible, should be avoided.
Furthermore, the use of such reagents not only adversely affects flotation behavior, it affects downstream operations such as dewatering and settling of the minerals. Additionally, and particularly with depressants, there is a requirement to add more reagent at each stage of the separation process.
In the second approach, a separate flotation system is dedicated to the recovery of the naturally floating mineral. Reagents are added to prevent the flotation of the valuable sulfide minerals, however with varying degrees of success. Inevitably, there will be at least some loss of the valuable mineral with the gangue recovered from the pre-flotation system. Such losses represent an economic disincentive and should ideally be avoided.
The applicants have previously attempted to address this problem by providing a pre-flotation treatment in which the major proportion of the non-sulfidic or naturally floating materials are separated from the valuable sulfidic mineral prior to the primary flotation step. In this process, which is subject of Australian patent application no 28746/95, a mineral slurry is subjected to a sequence of mineral dressing operations in which an inert gas and/or reducing agent are added to the slurry to maintain an electrochemical potential conducive to the separation of the minerals by flotation.
However, apart from the requirement of an additional pre-float stage, such pre-flotation may adversely affect the recovery of the valuable sulfidic mineral in the subsequent primary flotation step.
It has been previously reported that nitrogen, with and without organic depressants, may have an effect in the recovery of nickel. These previous disclosures, however, generally use nitrogen as a flotation agent to maximize sulfide flotation, e.g. pyrrhotite, pentlandite or pyrite which has nickel, cobalt or some precious metals associated therewith. Increasing quantities of depressants are required to provide effective separation of the nickel and pyrrhotite for example.
In an effort to ameliorate at least some of the disadvantages of the prior art it is proposed to provide a method for conditioning a slurry or flotation concentrate which improves the separation of valuable sulfidic minerals from non-sulfidic "gangue" material.